The little drummer boy from Las Vegas has become the year’s most unlikely rock star.

OK, Imagine Dragons frontman Dan Reynolds isn’t exactly little. He’s 6 feet 4. But his band is making an even bigger noise.

Imagine Dragons’ “Night Visions” is the second-biggest-selling rock album of 2013, behind Mumford & Sons’ Grammy-winning “Babel.” The single “Radioactive” has sold 4.4 million and spent 13 weeks at No. 1 on Billboard’s modern-rock chart, making it the fifth-biggest alt-rock hit of the past 25 years. The band’s three videos — last year’s “It’s Time,” “Radioactive” and the new “Demons” — have been viewed more than 100 million times on Vevo.

“We had no idea this would happen. We’re so used to being our own thing, building an organic audience slowly, like we’ve done for three years,” said Reynolds, whose band will close its U.S. tour Monday in St. Paul before heading off to Mexico, Australia and Europe. “I remember when ‘Night Visions’ was going to come out [last September], we said if it sold 10,000 the first week, we’d be ecstatic.”

Well, it sold 83,000 and debuted at No. 2 on Billboard’s album chart. After 53 weeks, it remains in the top 10.

To be clear, Reynolds isn’t the band’s drummer — Daniel Platzman is — but in concert the hyperkinetic frontman runs around the stage pounding on six drum setups, from a Japanese taiko to a concert bass drum. Is he channeling his inner marching-band drummer?

“I never did marching band,” Reynolds, 26, said last week from Orlando, Fla. “I played saxophone in jazz band. I had piano lessons from [age] 6 to 16. I did grow up playing drums — I took lessons for a few years — and played in high school garage bands. I was a drummer before I was ever a singer. I sing a lot more percussively because of that. I think growing up playing drums changes how you write melodies.”

Was he a hyperactive kid?

“100 percent! I have had ADD my whole life, depression, anxiety. I think that’s probably a common thread for most artists. I could never pay attention in class, always getting in trouble, always getting in detention for talking.”

But it wound up OK when Reynolds hooked up with some fellow musicians while attending Brigham Young University in Utah. After a couple of personnel changes and a move to his hometown of Vegas, Imagine Dragons ended up with a guitarist, drummer and bassist who all had studied at the prestigious Berklee College of Music.

Hip-hop producer

The band toiled for a couple of years in Vegas casinos, playing six hours a night, before hitting the road as indie-rock unknowns. After self-releasing three EPs, Imagine Dragons worked with big-time hip-hop producer Alex Da Kid, (Eminem, Nicki Minaj). On “Night Visions,” he helped them add hand claps and a big drum sound.

“We’ve always been rhythmically focused,” Reynolds said. “I grew up on urban radio: 2Pac, Biggie. Making that association with Alex was huge. He helped us take it to another level and maximize what we felt was the most important part of Imagine Dragons — rhythm. He really helped us get that kick drum sound or the snare to hit harder.”

They also added Vegas touches.

“You can’t help but be influenced by Vegas,” Reynolds said. “Lights, noise, nobody sleeps. We grew up playing in casinos, where you’re competing with slot machines and bikini blackjack dealers. So you create music that is a little larger than life — just like Vegas.”

Indeed, Imagine Dragons has a flair for anthemic arena rock, with shades of U2 and Coldplay, and even ambitious gimmicks such as Reynolds zip-lining over the crowd while playing a drum.

Missed daughter’s birthday

The band has been gigging so hard that Reynolds had to have a polyp removed from his vocal cords a year ago.

“It was from singing too much,” he said. “After that, we had to make rules: only three shows in a row and then a day off to recuperate. Even right now I’m on a steroid because we broke our rule of three in a row. We just couldn’t say no to this We Day show in Canada.”

Another casualty of their grueling schedule: The singer missed his daughter’s first birthday last month.

“I had to celebrate it with her a week and a half later,” he said. “I had a day off in Oregon. That’s where my wife’s family is from, so we celebrated there. It was a bummer for me. This will be my second Thanksgiving that I missed. But it’s like any job; you have to make sacrifices. My wife’s a musician, so she understands. But it’s been difficult — the hardest part of everything.”

But there have been some unforgettable rewards — such as meeting his idol Rivers Cuomo, of Weezer, who told Reynolds that he loved Imagine Dragons’ music, or playing to festival crowds of 50,000 to 100,000 people.

But nothing may have been more satisfying than the letter Reynolds received recently from his third-grade teacher, Mrs. Rivers.

“I hadn’t heard from her since third grade. She said: ‘I saw your band on TV.’ It was a sweet letter. I got in trouble back in the day, and now it’s come full circle.”

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